Laser marking method including raster scanning of rapidly rewritten liquid crystal mask

ABSTRACT

The invention relates to a laser marking method in which a laser beam is used for raster scanning of a liquid-crystal mask, on which a desired marking pattern is formed and the laser beam passing through the liquid-crystal mask gives a marking to an object. A marking pattern on the liquid-crystal mask is successively switched over to a subsequent marking pattern every scanning line, to reduce variation in transmissivity of the liquid-crystal mask, thereby obtaining marking with less unevenness of image.

TECHNICAL FIELD

The present invention relates to a method of laser marking wherein amarking pattern is marked onto an object such as an IC by means of alaser beam passing through a liquid crystal mask, by raster-scanning thelaser beam over this liquid crystal mask on which a desired markingpattern is displayed, and more particularly to a method of reducingimage unevenness by decreasing the variation of the transmissivity ofthe liquid crystal.

BACKGROUND ART

In a laser marking system using a liquid crystal mask, a requiredmarking pattern is displayed on the liquid crystal mask and this markingpattern is marked onto an object by irradiating the object with thelaser beam through the liquid crystal mask. The laser beam is made toscan over the liquid crystal mask by so-called raster scanning producedby main scanning and auxiliary scanning.

Japanese Patent Publication No. H. 2-268988 discloses a conventionallaser marking device of this type.

In this conventional art, as shown in FIG. 8, the liquid crystal mask isdivided into a plurality of blocks and the time required for the laserbeam to perform scanning from the starting point to the end point ismeasured beforehand for each block, and re-writing of the markingpattern is performed in unit of each division by detecting the lapsedtime after the commencement of scanning by means of a timer etc. Forexample, if the liquid crystal mask is divided into two divisions, laserscanning of the lower division is performed after completion of laserscanning of the upper division and the marking pattern displayed in theupper division is concurrently switched over to the subsequent markingpattern.

In this way, re-writing of the liquid crystal mask and laser scanningare carried out in parallel, so the time required for the markingprocess can be shortened compared with the case where changeover of thedisplay pattern of the liquid crystal mask to the subsequent markingpattern is only effected after completing the laser scanning of theentire liquid crystal.

However, in the conventional art described above, the divided blockunits are of comparatively large area so variation in the transmissivityand temperature occurs between liquid crystal pixels in the same dividedblock, causing unevenness of the image produced by marking.

In more detail, even within the same block, there is a considerabledifference between the upper region and the lower region in the timethat elapses from the time-point of changeover of the liquid crystaluntil scanning is performed; this time difference causes unevenness oftransmissivity. For example, in FIG. 8 referred to above, consideringliquid crystal pixels a and b of block A, although the time-point atwhich the marking pattern of these pixels a, b is switched over is thesame, since the laser is scanned in raster scanning from the top left tothe bottom right, the time-point of the laser scanning in respect ofpixel a is considerably earlier than the time-point of the laserscanning in respect of pixel b. This therefore results in a largedifference between pixels a and b as regards the lapsed times from thetime-point of liquid crystal changeover until laser scanning isperformed; this causes variation of the transmissivity.

FIG. 9 shows the liquid crystal transmissivity characteristic. As can beseen from this Figure, the liquid crystal transmissivity reaches aprescribed transmissivity on the lapse of a prescribed time after thedrive voltage is turned on, but thereafter gradually rises as the timefor which the voltage is kept on gets longer.

Thus, by making the time for which the liquid crystal is maintained inthe on condition after the voltage being turned on fixed as far aspossible, the transmissivity of the liquid crystal can be fixed.

With the foregoing in view, it is an object of the present invention toprovide a laser marking method wherein the variation of transmissivityof the liquid crystal is reduced so as to enable marking to be performedwith little unevenness of the image.

DISCLOSURE OF THE INVENTION

According to the present invention, in a laser marking method in whichraster scanning of a laser beam is performed over a liquid crystal maskon which a required marking pattern is displayed, so that an object ismarked by the laser beam passing through the liquid crystal, the markingpattern of the liquid crystal mask is successively switched over to thesubsequent marking pattern in unit of a scanning line at which thescanning of the laser beam is completed.

That is, according to the present invention, changeover of the displaypattern of the liquid crystal mask is effected in unit of a singlescanning line for every scanning line at which the scanning of the laserbeam is completed.

Also according to another aspect of the present invention, in a lasermarking method in which raster scanning of a laser beam is performedover a liquid crystal mask on which a required marking pattern isdisplayed, so that an object is marked by the laser beam passing throughthe liquid crystal, the marking pattern of the liquid crystal mask issuccessively switched over to the subsequent marking pattern in unit ofa liquid crystal pixel at which the scanning of the laser beam iscompleted.

That is, according to another aspect of the present invention,changeover of the display pattern of the liquid crystal mask is effectedin unit of a single pixel for every liquid crystal pixel at which thescanning of the laser beam is completed.

Consequently, with the present invention, since laser scanning isperformed with the liquid crystal display pattern being switched overwith unit of one main scanning line or one pixel, variation oftransmissivity of the pixels of the liquid crystal mask can be reduced,enabling marking to be performed with little unevenness of the image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing an embodiment of the present invention;

FIG. 2 is a diagram showing the overall construction of a laser markerdevice;

FIG. 3 is a view showing raster scanning;

FIG. 4 is a view showing changeover of the marking pattern with a singlescanning line as unit;

FIG. 5 is a flow chart showing a further embodiment of the presentinvention;

FIGS. 6(a), 6(b) and 6(c) are views given in explanation of theembodiment of FIG. 5;

FIG. 7 is a flow chart showing a further embodiment of the presentinvention;

FIG. 8 is a view given in explanation of the conventional art; and

FIG. 9 is a view showing the change in the liquid crystal transmissivitywith time.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described in detail below with reference toembodiments shown in the accompanying drawings.

FIG. 2 shows an example of the overall construction of a laser markerdevice to which the method of the present invention is applied. Laserlight generated by a laser oscillator 1 is directed onto a liquidcrystal mask 6 through a Y-direction deflector constituted by a scannermirror 2, a lens 3, an X-direction deflector constituted by a polygonalmirror 4, and a lens 5. With this construction, raster scanning of thelaser light over liquid crystal mask 6 is performed by means of theauxiliary scanning (Y-direction) achieved by scanner mirror 2 and themain scanning (X-direction) achieved by polygonal mirror 4.

Controller 7 controls the raster scanning over liquid crystal mask 6 bycontrolling a motor 8 for the scanner mirror and a motor 9 for thepolygonal mirror and also controls the laser oscillation generated bythe laser oscillator 1. In addition, controller 7 executes displaychangeover control of the marking pattern on liquid crystal mask 6.

The laser light that has passed through liquid crystal mask 6 isdirected onto object 13 through Y-direction detector constituted bymirror 10, lens 11, and X-direction deflector constituted by lens 12equipped with a moving table; the marking pattern that is displayed onliquid crystal mask 6 is thereby marked onto object 13. The Y-directiondeflector constituted by mirror 10 deflects the laser light in the Ydirection by being driven in rotation by motor 14 and the X-directiondeflector constituted by lens 12 deflects the laser light in the Xdirection by displacement of moving table 16 produced by motor 15. Thismirror 10 and lens 12 are provided to effect positional alignment of thelaser beam with respect to object 13, positional alignment control ofthese components being performed by controller 7.

In this construction, controller 7 executes laser scanning control andmarking pattern display changeover control as shown by the flow-chart ofFIG. 1.

First of all, by driving liquid crystal mask 6, controller 7 displays onliquid crystal mask 6 the marking pattern with which marking is to beinitially effected (step 100)

Next, controller 7 turns laser oscillator 1 on and commences rasterscanning of the laser light as shown in FIG. 3 by controlling motor 8and 9 (step 110).

After this, when controller 7 detects completion of raster scanning ofone main scanning line (step 120), raster scanning shifts to thesubsequent main scanning line and liquid crystal mask 6 is driven (step130) such as to switch over the marking pattern in respect of the mainscanning line at which the raster scanning is completed to the markingpattern that is to be displayed next.

Subsequently in the same way, every time controller 7 completes rasterscanning of one main scanning line, raster scanning is shifted to thesubsequent main scanning line and liquid crystal mask 6 is driven (step140-step 130) such as to switch over the marking pattern in respect ofthe main scanning line at which the raster scanning is completed to themarking pattern that is to be displayed next.

After this, when laser scanning of the present marking pattern iscompleted (step 140), controller 7 switches the marking position on theobject by driving motors 14 and 15, and raster scanning of liquidcrystal mask 6 is again performed, thereby marking the object with thesubsequent marking pattern (step 150).

The same control is executed repeatedly until all marking has beencompleted (step 160).

FIG. 4 shows how the marking pattern of the liquid crystal mask isswitched over in unit of a single main scanning line. Changeover from aninitial scanning pattern "A" to the subsequent scanning pattern "B" inunit of one main scanning line is shown.

FIG. 5 shows another embodiment of the present invention, in which astep 145 is inserted between step 140 and step 150 of the flow chartindicated in FIG. 1.

Specifically, in this embodiment, on completion of laser scanning of agiven marking pattern and before starting laser scanning of thesubsequent marking pattern, the liquid crystal drive voltage of theliquid crystal panel is temporarily turned off for a prescribed time,the liquid crystal being driven in accordance with the subsequentmarking pattern only after this prescribed off time has elapsed.

For example, when the display content is switched over to a pattern asshown in FIG. 6(b) from the pattern shown in FIG. 6(a), with theconventional system, the two pixels indicated by "*" would besuccessively in the on condition, resulting in the difference intransmissivity from that of other portions so as to give rise tounevenness of the image. In this embodiment, on the other hand, as shownin FIG. 6(c), the transmissivity of the pixel elements is madepractically uniform by inserting a step in which the drive voltage ofthe entire device is turned off for a prescribed time, thereby reducingunevenness of the image. More specifically, in this embodiment, themarking pattern is switched over in the order (a)→(b)→(c)→(b).

FIG. 7 shows a still another embodiment of the present invention. Inthis embodiment, the display of the marking pattern is switched over inunit of one liquid crystal pixel.

Specifically, first, controller 7 drives liquid crystal mask 6 todisplay thereon the marking pattern with which marking is to beinitially effected (step 200).

Next, controller 7 turns laser oscillator 1 on and controls the drive ofmotor 8 and 9 so as to commence raster scanning by the laser light asshown in FIG. 3 (step 210).

After this, when controller 7 completes laser scanning of one liquidcrystal pixel (step 220), laser scanning shifts to the subsequent pixeland liquid crystal mask 6 is driven such as to switch over the markingpattern in respect of the one pixel whose laser scanning has just beenperformed to the marking pattern that is to be displayed next (step230).

Subsequently in the same way, every time controller 7 completes laserscanning of one pixel, laser scanning is shifted to the subsequent pixeland liquid crystal mask 6 is driven such as to switch over the markingpattern in respect of the pixel at which the laser scanning is completedto the marking pattern that is to be displayed next (step 240-step 230).

After this, when laser scanning of the present marking pattern iscompleted (step 240), controller 7 switches the marking position on theobject by driving motors 14 and 15, and raster scanning of liquidcrystal mask 6 is again performed, thereby marking the object with thesubsequent marking pattern (step 250).

The same control is executed repeatedly until all marking has beencompleted (step 260).

It should be noted that, even in the embodiment of FIG. 7, as in theembodiment of FIG. 5, it would be possible to insert a voltage off step,i.e. a step in which the liquid crystal drive voltage is turned off fora prescribed time, between step 240 and step 250.

Also, although in the above embodiment it was arranged for the markingpattern to be switched over with a unit of one pixel, it would bepossible to switch over the marking pattern in unit of a few pixels or afew tens of pixels or a few hundred pixels.

INDUSTRIAL APPLICABILITY

In a method of laser marking in which a marking pattern is marked ontoan object such as an IC by a laser beam that passes through a liquidcrystal mask, the present invention is effective in making it possibleto obtain marking with little unevenness of the image.

We claim:
 1. A method of laser marking in which a laser beam israster-scanned over a liquid crystal mask on which a required markingpattern is displayed and an object is marked by the laser beam that haspassed through the liquid crystal mask, wherein each scanning line ofthe liquid crystal mask is scanned by the laser beam while displaying afirst marking pattern, each scanning line being switched to display asecond marking pattern directly after being scanned by the laser beam.2. A method of laser marking according to claim 1, in which oncompletion of changeover of the marking pattern of the liquid crystalmask of the subsequent pattern and before starting laser beam scanningof the subsequent pattern, a liquid crystal drive voltage is temporarilyturned off for a prescribed time, and thereafter the liquid crystal isdriven in accordance with the subsequent pattern.
 3. A method of lasermarking in which a laser beam is raster-scanned over a liquid crystalmask on which a required marking pattern is displayed and an object ismarked by the laser beam that has passed through the liquid crystalmask, wherein each scanning line of the liquid crystal mask is scannedby the laser beam while displaying a first marking pattern, eachscanning line being switched to display a second marking pattern in theform of a liquid crystal pixel after being scanned by the laser beam. 4.A method of laser marking according to claim 3, in which on completionof changeover of the marking pattern of the liquid crystal mask of thesubsequent pattern and before starting laser beam scanning of thesubsequent pattern, a liquid crystal drive voltage is temporarily turnedoff for a prescribed time and thereafter the liquid crystal is driven inaccordance with the subsequent pattern.